Hi,
Since your IF-THEN statements are actually GOTOs (if evaluated true) all you need to do is add GOSUB (or GOTO) WRONG after the three IF statements.
Code:
IF Serial[1]=$15 AND Serial[2]=$D3 AND Serial[3]=$7D AND Serial[4]=$16 AND Serial[5]=$00 AND Serial[6]=$00 THEN correct1    '<----This will GOTO correct1
IF Serial[1]=$68 AND Serial[2]=$36 AND Serial[3]=$4F AND Serial[4]=$17 AND Serial[5]=$00 AND Serial[6]=$00 THEN correct2
IF Serial[1]=$1E AND Serial[2]=$1A AND Serial[3]=$2C AND Serial[4]=$17 AND Serial[5]=$00 AND Serial[6]=$00 THEN correct3
GOSUB WRONG
GOTO mainloop
Now the WRONG subroutine will only execute if none of the three IF statements evalutate true.

/Henrik.